The present invention relates to D.C. machines having a plurality of field poles each of which comprises a permanent magnet and an auxiliary pole juxtaposed to the permanent magnet and made of a magnetic material exhibiting a reversible permeability higher than that of the permanent magnet.
D.C. motors having field poles made of permanent magnets and auxiliary poles are known as disclosed in Japanese patent Laid-Open No. 57-153558/1982. The D.C. motor disclosed therein has a plurality of field poles each consisting of a permanent magnet and an auxiliary pole which are juxtaposed circumferentially on the inner peripheral portion of a yoke. The auxiliary pole is made of a magnetic material exhibiting a reversible permeability higher than that of the permanent magnet, and is arranged at the side of the permanent magnet wherein the magnetic field flux is increased by an armature reaction.
The D.C. machine provided with such auxiliary poles has the advantage that, owing to the arrangement of each auxiliary pole on the side of the magnet in which the magnetic field flux is increased by the armature reaction, an increased magnetic flux effectively passes through an armature core, thereby to heighten a torque.
There have been proposed various techniques for further enhancing the performances of the D.C. machines, for example, through improvements in dimensional relationship between the permanent magnet, the auxiliary pole and teeth and slots of an armature. Examples of those techniques are as follows:
A D.C. machine disclosed in Japanese Utility Model Laid-Open No. 58-153573 (1983) is provided with permanent magnets and auxiliary poles in which the width of each of the permanent magnets has a width twice as wide as one of an auxiliary pole, whereby performance of the D.C. machine is enhanced.
Japanese Utility Model Laid-Open No. 59-34485 (1984) discloses a D.C. machine in which the length of air gap defined between a permanent magnet and an auxiliary pole is made shorter than the width of a slot opening.
Further, there is Japanese patent Laid-Open No. 61-73563 which is concerned with a fixing structure in a D.C. motor in which a permanent magnet and an auxiliary pole are fixed to a yoke by a sleeve having concave and convex portions. Although this Japanese Patent Laid-Open does not have therein any description on a dimensional relationship between tooth width or tooth pitch of an armature and the width of the permanent magnet or the auxiliary pole, it seems from an illustration of the fixing structure of the permanent magnet and the auxiliary pole to the yoke that the dimensional relationship between the permanent magnet, the auxiliary pole and tooth pitch or width is taken into consideration.
Further, in a D.C. machine for conducting a high current and attaining a high torque, such as a starting motor for automobiles, a large number of thick coils need to be wound on an armature, which naturally increases the number of slots. For example, the number of the coils per slot is 2-6 or so, and the number of the slots per pole is at least 4 (in general, 5 to 9). Thus, the torque is heightened by increase in the total number of the coils and the resultant increase in the amounts of magnetic fluxes.
As mentioned above, various techniques have been used for attaining high performance of D.C. machines. However, it is still required to attain further high performance of the D.C. machines without increasing size of the machine.